We investigated the expression of granulocyte colony-stimulating factor (G-CSF), G-CSF mRNA, and G-CSF receptor mRNA in astrocytoma cell lines, G-CSF in astrocytoma cyst fluid, and the effect of recombinant G-CSF on the proliferation of astrocytoma cells in vitro and in vivo. We first examined supernatants from astrocytoma cell lines for the presence of G-CSF by ELISA. G-CSF was expressed by 6 of 14 astrocytoma cell lines constitutively, and, was detected after stimulation with tumor necrosis factor-alpha (TNF-alpha) in four of eight cell lines which did not produce G-CSF constitutively. G-CSF mRNA was detected by reverse-transcriptase polymerase chain reaction (RT-PCR) in all cell lines studied, suggesting that astrocytoma cells have the potential to produce G-CSF. We also analyzed the presence of G-CSF by ELISA in five astrocytoma cyst fluids. G-CSF was detected in one case. Although, in vitro study, the growth of glioma cells was not affected by rG-CSF, in a mouse model, the administration of G-CSF significantly shortened the time to tumor appearance and accelerated tumor growth. These data suggest that G-CSF has a stimulatory effect on the proliferation of astrocytoma cells in vivo through the mediation of host factors.